Pulverized fuel combustion implies a method in which the fuel, i.e., coal in conventional combustion but also peat to an increasing extent, is milled into a very fine-grained dust, which is then blown to the boiler via a nozzle using stack flue gas or air as the carrier. In coal- and peat-fired power plants, pulverized fuel combustion is a common method of combustion which inherently merits an extremely high value to improvements in the ignition and combustion of pulverized fuel.
Monitoring of the combustion process is availed to reduce the proportion of expensive auxiliary fuels. The monitoring operation is implemented in several ways, of which optical flame detectors are gaining ground thanks to the large information available from them.
A conventional method of monitoring combustion in a burner is to use a video camera, often called a fire-box camera. The video camera that produces a black-and-white or color video signal is located in a heat-resistant and cooled protective tube. In addition to air cooling, some cameras are provided with water cooling. The camera installations are generally provided with an automatic protection that ejects the camera out from the fire-box when a system malfunction is encountered.
Furthermore, flame monitoring is implemented with pyrometers sensitive to radiation intensity as well as with other types of detectors tuned to a narrow band of wavelengths. The quality of the combustion process is evaluated on the basis of flame instability (from the "DC" and "AC" components of flame intensity). A more advanced version of the aforementioned method is the cross-correlation method, also called the incremental volume method.
Use of a camera in the conventional methods is restricted to the monitoring of the averaged combustion process. The operation of a single burner can be monitored only at the ignition of the first flames and the extinction of the last flames. Detectors of the pyrometer category are hampered by such factors as placement and alignment of the detector, low temperature of the flame, etc. Some types of detectors are prone to erroneous response to nearby flames and background radiation from the walls of the fire-box. A disadvantage of the cross-correlation method is, for instance, its high sensitivity to changes in burning rate.